Can not select object in other collection
This is a classic gotcha.

If you see this in the 3D viewport ...



You're still in Edit Mode and although it looks like you've selected another object, you really haven't ... you've added a secondary object to the selection.

Hit the TAB key to go back to Object Mode, then you should be able to select objects normally again.

Alternatively if you want to stay in edit mode but switch objects then you can simply click the Edit Mesh toggle in the outliner.



This is a real pain in the -Z, something I hope is addressed in the future.  It catches me out daily.